The known methods and apparatii for oxygen delignification of medium consistency pulp slurry consist of the use of high shear mixers and upflow pressurized reactors with retention times of twenty to sixty minutes. These are operated at consistencies of ten to fourteen percent (o.d.) at an alkaline pH of from 10.5 to 13. Oxygen gas is contacted with the pulp slurry in a turbulent state lasting less than one second. These have evolved to processes and apparatii using two pressurized reactors, each with high shear mixers, to mix the oxygen gas twice, to improve overall performance. To date, use of the aforesaid methods and apparatii have typically resulted in pulp kappa reductions (i.e., delignification) of forty to forty-five percent, with some two-reactor systems claiming more than forty-five percent. However, many systems perform below forty percent kappa reductions.
The disadvantages of the known methods and apparatii is that the low levels of kappa reduction make medium consistency oxygen delignification, by itself, unacceptable as a pretreatment to a Total Chlorine Free (TCF) bleach plant utilizing ozone and peroxide bleaching agents. TCF bleach plants are documented as requiring incoming kappa numbers below fifteen, and preferably below twelve. These low kappa numbers are required for reasons of quality, economics, process design, and such. Process technology required to achieve these low kappa results for softwoods, in addition to medium consistency oxygen delignification, are quinone (AQ) cooking. It has also been claimed (U.S. Pat. Nos. 5,173,153 and 5,085,734) that the high consistency oxygen delignification with the patented O.sub.M process results in reduction of sixty percent, and is the preferred oxygen delignification technology.
These aforesaid technologies require high capital expenditures or high consistency oxygen delignification processing for pulp treatment before the TCF bleach plant, and accordingly, will exclude many pulp mill operations from the ability to economically modify their processes. In most cases they also require the installation of significant amounts of equipment which have a high level of operational complexity. In addition, there is still a penalty in product yield associated with the extended cooking to attain the kappa levels necessary for TCF bleaching.
It has been understood that oxygen delignification reaction proceeds under two distinct orders of reaction kinetics. The first reaction occurs rapidly, and is responsible for lignin fragmentation (delignification). It is a radical bleaching reaction that is dependent on alkali concentration or pH to proceed. It also consumes alkali as it proceeds and generates organic acids, causing pH to drop by one to two points during the reaction time. This is consistent with the field observations of operating systems. The second reaction occurs slowly, at a rate estimated to be twenty times slower than the first reaction. This reaction is responsible for the destruction of chromophoric structures (brightness development). It is an ionic bleaching reaction that is dependent on alkali concentration, or pH, to proceed. It also will consume alkali as it proceeds and generate organic acids, causing the pH to drop by one to two points during the reaction time.